Ultrasonic echoscopy provides information about an examined object which may be displayed in the form of an ultrasonic echogram. Such an echogram consists of a display of acoustic impedance discontinuities or reflecting surfaces in the object. It is obtained by directing a short pulse of ultrasonic energy, typically having a frequency in the range of from 1 to 30 MHz, into the object being examined. Any acoustic impedance discontinuities in the object reflect and return some of the energy in the form of an echo. This echo is received, converted into an electrical signal and displayed as an echogram on a cathode ray oscilloscope, a film, a chart or the like.
The echogram may constitute a one dimensional (A-mode) or a two dimensional (B-mode) representation of the object being examined. In both cases, the information is contained in the position and magnitude of the echo displayed. In a one dimensional display, the position along a base line is used to indicate the distance to the reflecting surface whilst the magnitude of the echo is displayed, for example, as a deflection of the base line or as an intensity change. In a two dimensional display, the position along a base line is used to indicate the distance to the reflecting surface (as in a one dimensional display) and the direction of the base line is used to represent the direction of propagation of the acoustic energy. The two dimensional display is obtained by changing this direction of propagation of the acoustic energy and by instituting a similar, but not necessarily identical, movement of the base line of the display. The magnitude of the echo is displayed as for a one dimensional display, for example, as a deflection of the base line or--more usually--as an intensity change.
The technique of ultrasonic echoscopy used in medical diagnosis to obtain information about the anatomy of patients has been widely reported. It has proved of particular value as a diagnostic aid when examining the abdomen and uterus, eye, breast, lung, kidney, liver and heart, these being regions of soft tissue with little bone and air. In general, the technique is considered to compliment other techniques to provide a more complete picture of a patient's condition. However, particularly in pregnancies, ultrasonic echoscopy may be useful in place of x-rays where the latter may not give sufficient information or may be dangerous. In such medical applications, a pulse of ultrasonic energy is transmitted into a patient in a known direction and echoes are received from reflecting surfaces within the body. The time delay between the transmission of a pulse and the reception of an echo depends on the distance from the transmitting ultrasonic transducer to the reflecting surface. The distance information so obtained may be displayed for interpretation and clinical use as a one dimensional range reading or as a two dimensional cross section, as previously described.
It is known (see, for example, the specification of U.S. Pat. No. 3,939,707 to Kossoff) to measure blood flow in the body along an ultrasonic line of sight by measuring the frequency shift of ultrasonic echoes and combining the frequency shift data with blood vessel dimensional and directional information that has been obtained from the B-mode ultrasonic echogram of the area. Using this technique, an absolute measurement of the blood flow is obtained.
It is assumed in the simple application of the pulse echo principle that a pulse of ultrasonic energy propagates through the various media of an object at a uniform velocity of propagation. In soft tissues in a human body, this velocity of propagation is of the order of 1570 meters per second (m/s). However, a scanning beam of ultrasound will suffer distortion as it propagates through media having different characteristics from those of the soft tissue being examined. The distortion is due to refraction, which is a consequence of the different velocity of propagation in each medium, and attentuation (which is due to a number of effects, including reflection at the interfaces between media and scattering). The distortion is manifest as an unclear or inaccurate echogram of the object being examined, and in deviation of the beam from its initial line of sight, widening of the beam and an increase in the level of the sidelobes of the transmitted signal.
It has long been recognised that the removal of these distortions will give an improvement in the resolution and clarity of the resulting echograms. An improvement in the beam quality will also improve the accuracy of tissue characterisation measurements and will improve the signal to noise ratio for the ultrasonic Doppler technique used to measure blood flow.